A driving force in microprocessor design is to increase the speed at which computer instructions are executed. One approach is the multi-core design, which includes multiple central processing units (CPUs) in a single microprocessor circuit. Another approach is to design CPUs with multiple internal functional units that can be used in parallel. For example, various multi-thread microprocessors can allow several independent threads of program instructions to be executed concurrently, sharing the internal resources of one CPU.
In CPUs designed for simultaneous multi-threading (SMT), several components of a CPU can be used simultaneously to execute several instruction threads. In various situations, SMT can provide efficient use of microprocessor resources. However, many SMT designs suffer from problems that arise during execution time when instruction threads are being processed by the CPU. One issue is the fairness of real-time resource allocation of the CPU resources among the various threads. A related issue is uncertainty in execution time for individual threads. In various situations, one thread may be “starved” for an undesirably long time while one or more resources on a CPU are assigned to different thread. Such delays can make a thread's execution time unpredictable. This unpredictability can pose problems for thread management systems, and can complicate the task of software design.